The present invention relates to furnaces in which fuel and oxidant are injected into a combustion zone.
A combustion furnace generates heat by the combustion of fuel with an oxidant. The fuel is typically natural gas, and the oxidant is typically air, vitiated air, oxygen, or air enriched with oxygen. These reactants are injected into a combustion zone within the furnace. Combustion of the fuel and oxidant in the combustion zone causes oxides of nitrogen to result from the combination of oxygen and nitrogen. It is sometimes desirable to reduce the production of NOx.
The invention provides a method and apparatus for injecting fuel and preheated oxidant into a combustion zone at target rates of injection.
In accordance with the method, primary fuel and preheated oxidant are injected into the combustion zone to produce a flame with a predetermined adiabatic flame temperature. The preheated oxidant is injected at the target rate of oxidant injection. However, the primary fuel is injected at a first reduced rate which is less than the target rate of fuel injection. Secondary fuel is simultaneously injected into the combustion zone separately from the flame at a second reduced rate which is equal to the difference between the first reduced rate and the target rate of fuel injection. In this manner, the invention enables combustion of the fuel with the preheated oxidant to provide the amount of heat expected from the target rates of injection, while maintaining an adiabatic flame temperature that is lower than it might otherwise be if the target rate of fuel injection were provided entirely at the flame. The lower adiabatic flame temperature provides a correspondingly lower rate of NOx production.
The apparatus includes a controller which is operative to receive an input indicative of the temperature of the preheated oxidant. The controller responds to the temperature input by identifying an oxidant-to-fuel ratio at which combustion of the fuel with the preheated oxidant can occur at the predetermined adiabatic flame temperature. The value of the preheated oxidant in the identified ratio is equal to the target rate of oxidant injection. The value of the fuel in the identified ratio is a first reduced rate which is less than the target rate of fuel injection. Additionally, the controller is further operative in response to the temperature input to determine a second rate of fuel injection which is equal to the difference between the first reduced rate and the target rate of fuel injection.
Further in accordance with the invention, the apparatus includes a device which is operative to sense the temperature of the preheated oxidant, and a reactant supply system which is operative in response to the controller to inject the primary fuel and the preheated oxidant into the combustion zone at the identified ratio to produce the flame with the predetermined adiabatic flame temperature. The reactant supply system simultaneously injects secondary fuel into the combustion zone separately from the flame at the second reduced rate. Preferably, the reactant supply system injects the primary fuel and the preheated oxidant into the combustion zone together as fuel lean premix, and injects the secondary fuel into the combustion zone at a plurality of separate locations. The second reduced rate is the sum of the rates at which the secondary fuel is injected at the separate locations.